1. Field of the Invention
The present invention relates to a method for producing a zinc oxide-based semiconductor light-emitting device and a zinc oxide-based semiconductor light-emitting device; and particularly relates to a method for producing a zinc oxide-based semiconductor light-emitting device by layering a plurality of semiconductor layers on a substrate for growing zinc oxide-based material, and to a zinc oxide-based semiconductor light-emitting device produced by the above-noted production method.
2. Description of the Related Art
Zinc oxide (ZnO) is a direct band gap semiconductor having a band gap energy of 3.37 eV at room temperature and holds promise as a material for use in optical devices in the blue-ultraviolet region. In particular, zinc oxide possesses physical properties that are especially suitable for semiconductor light-emitting devices; i.e., an exciton binding energy of 60 meV and a refractive index (n) of 2.0. Furthermore, zinc oxide is inexpensive in terms of raw materials and has no adverse effects on the environment or the human body. Based on such physical properties and characteristics, there are high expectations that a high-efficiency, low-power consuming semiconductor light-emitting device can be achieved using zinc oxide-based materials.
Conventionally, a monometal is used as an electrode of a zinc oxide-based semiconductor light-emitting device. For example, aluminum (Al) is used as an electrode formed on a growth substrate (e.g., Japanese Patent Kokai No. 2004-95649 (Patent Literature 1) and WO 2007/015330 (Patent Literature 2). In Patent Literature 2, an n-type ZnO substrate as the growth substrate is ground and an electrode composed of Al is formed on the ground surface. However, there are problems in that oxide crystals and metal bond poorly together and sufficient ohmic contact cannot be obtained.
Alloys that include Ti are used in zinc oxide-based semiconductor light-emitting devices because of this bonding problem. For example, electrodes composed of Ti/Au, Ti/Al, or Ti/Al/Au alloy are known to be in use (e.g., Japanese Patent Kokai No. 2009-43913 (Patent Literature 3) and Japanese Patent Kokai No. 2009-160025 (Patent Literature 4). However, there are problems in that in electrodes composed of a Ti/Au alloy, Au absorbs emitted light; in electrodes composed of a Ti/Al alloy, the Al surface oxidizes and produces defective connections and defective joining; and in electrodes composed of a Ti/Al/Au alloy, an alloy layer having high electrical resistance is formed on the Al/Au interface. Furthermore, there is a problem in that the use of Ti leads to a reduction in reflectivity.